The present invention relates to control circuits for activating electrical loads of electrical appliances.
Control systems for the activation of electrical loads in electrical appliances are known which comprise an electronic circuit that includes logic control means such as a microprocessor. It is important to guarantee that this type of control systems are safe against failures of the electronic circuit itself, and particularly against failures of the logic control means.
U.S. Pat. No. 5,175,413 discloses a failsafe control system for operating a power relay to energize a resistive heating element in a cooking apparatus. The control system comprises a logic circuit, such as a microprocessor, with an output connected to a first branch which acts on a first transistor and a second branch which acts on a second transistor. The first and second transistors are both in series, and are also in series with the power relay. The first branch, which comprises an operational amplifier, only acts on the first transistor if it receives pulses from the microprocessor output, so that the power relay is not energized if the microprocessor fails. The second branch, which also comprises an operational amplifier, acts on the second transistor in accordance with the user's instructions.
The main drawback of the control system disclosed is that it is not failsafe against the sticking of the power relay contacts which control the load, as said sticking would make said load be switched on uninterruptedly.
Furthermore, in said control system it is necessary to use high-capacity transistors against the second break, as if a failure occurred in the free-wheeling diode of the power relay, said diode remaining in open circuit, the overvoltage that would occur in said transistors on interrupting the current of the power relay coil would be very high. If an avalanche occurred in said transistors due to the overvoltage generated by the power relay coil, the transistors would short-circuit, meaning that, with a single failure, the power relay contacts would be closed.
Furthermore, it is necessary that the two operational amplifiers are not integrated in the same integrated circuit since, for example, with a single failure such as the ground disconnection of said integrated circuit, both outputs would be simultaneously at a high level, causing current to flow through the power relay coil, the load being energized uninterruptedly.